1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly it relates to a liquid crystal display device in which ferroelectric liquid crystals are used.
2. Description of the Related Art
In the conventional liquid crystal display devices, some of them use nematic liquid crystals while others use ferroelectric ones. Examples of liquid crystal display devices using nematic liquid crystals are those of twisted nematic type (TN type) and supertwisted birefringence effect type (SBE type).
In the liquid crystal display device of TN type, there is a disadvantage with a progress of multiplexing of a driving means that driving margin becomes narrower whereby sufficient contrast is not resulted. Further, in the liquid crystal display device of SBE type with great twist angles which is an improved version of that of TN type, there is a disadvantage that, when it is used for display of large capacity, contrast lowers and response speed becomes slower.
In order to improve such disadvantages, N. A. Clark and S. T. Lagerwall proposed a liquid crystal display device using chiral smectic C liquid crystals, i.e. ferroelectric liquid crystals (cf. Japanese Laid-Open Patent Publication 56/107216 and U.S. Pat. No. 4,367,924).
Unlike the nematic liquid crystal display devices in which rotating power generated only by polarity of electric field using dielectric anisotropy of nematic liquid crystals, the above liquid crystal display device is a ferroelectric one using rotating power generated by spontaneous polarization of ferroelectric liquid crystals and that of electric field. Characteristic features of the liquid crystal device are bistability, memory and quick response. Thus, as shown in FIG. 6, when the ferroelectric liquid crystals are injected in a cell in which the gap is made thin, helical structure of the ferroelectric liquid crystals gets loose as a result of interfacial affection and an area in which the liquid crystal molecules 18 are in a stable state with an angle of inclination of +.theta.19 to the normal line 17 of smectic layer and another area in which they are in a stable state with an angle of -.theta.20 to the opposite direction are mixed whereby a bistability is resulted. When electric field 16 is applied to the ferroelectric liquid crystals in said cell, directions of the liquid crystal molecules 18 and spontaneous polarization thereof 15 can be uniformly aligned and, upon changing the polarity of the applied electric field 16, switching of orientation of the liquid crystal molecules 18 from one state to another one is possible.
As a result of said switching driving, there is a change of light of double refraction in the ferroelectric liquid crystals in the cell and, therefore, when the cell is sandwiched between two polarizers, it is possible to control the transmitted light. Further, even when the applied electric field is canceled, orientation of the liquid crystals can be kept in a state prior to the cancel of application of field due to the controlling ability of the interface for the orientation whereby memory effect can be resulted too. In addition, the time necessary for the switching driving is as little as 1/1,000 or less of that of liquid crystal display device of twisted nematic type because of a direct action of electric field with spontaneous polarization of liquid crystals whereby quick response is possible.
Thus, construction of liquid crystal display devices with many scanning lines and high degree of resolution by a multiplex driving system have been attempted already by utilizing the memory effect and the quick response of such ferroelectric liquid crystals.
However, even in the liquid crystal display device of the type by Clark and Lagerwall, there are several problems. First, as compared with common nematic liquid crystals, the liquid crystals of chiral smectic C phase exhibiting ferroelectric property are less symmetric and higher ordered from the aspect of crystallinity. Accordingly, it is difficult to make them uniformly orientated whereby there is a disadvantage that homogeneous element is hardly prepared.
In initial model, the structure of smectic C phase layers is thought, as shown in FIG. 2, to be vertical to the substrates called a bookshelf type. In the drawing, 9 and 10 are substrates, 13 is a smectic layer and 17 is a direction of normal line of the smectic layer. When, however, cells are prepared using a conventional orientation film, there are large differences between the expected switching phenomenon as well as optical characteristics and the actual ones whereby entirely different switching has been found to take place from the proposed model.
One of the causes is that the layer structure is in a V-shaped bent state called chevron, as shown in FIG. 3, as a result of small angle scattering of x-ray [cf. T. P. Rieker, N. A. Clark, et al: Phys. Rev. Lett., 59, page 2658 (1987)]. In the drawing, 9 and 10 are substrates, 13 is a smectic layer and 14 is a juncture of the chevron structure. Another point which is different from the initial model is as follows. Thus, not only that the direction of spontaneous polarization and liquid crystal molecules have uniform orientations, but also that the molecules form a twisted orientation between the upper and the lower substrates [cf. M. Glogarova and J. Pavel, J. Phys. (France), 45, pages 143 (1984)].
It has been in the meanwhile found that the ferroelectric liquid crystals which are oriented especially by means of orientation film wherein a uniaxial orientation is conducted by rubbing are in a twisted orientation because of strong controlling force at the interface. It has been found that, in such orientation, difference in optical molecular axis in switching between the two states does not appear effectively in general whereby high contrast is not achieved. In order to solve such a disadvantage, there are several proposals in which the model initially proposed by Clark, et al. is achieved. As one of such proposals, there is a report that, in orientation film using a SiO rhombic chemical vapor deposition method, relatively high pretilt is given to the substrate interface whereupon bending of the layer can be prevented to achieve the obliquely inclined layer structure.
Second, there is a proposal on a method in which alternating field of high voltage is applied to a cell having a bent structure whereby the layer structure is changed to a bookshelf structure [cf. Sato, et al: The Twelfth Liquid Crystal Symposium (Nagoya), 1F16 (1986)] and the report said that high contrast characteristics were resulted in all cases. However, in the first-mentioned rhombic chemical vapor deposition method, there are large problems in terms of production that the technique for making chemical vapor deposition angle uniform is difficult and that it has a process in vacuo. In the method of applying electric field, it is difficult to change the layer structure homogeneously and, with an elapse of long time, there are many cases of returning to chevron structure. Thus, the method has not been practically used yet.